1Review Checklist for RCU Patches
2 3 4This document contains a checklist for producing and reviewing patches
5that make use of RCU. Violating any of the rules listed below will
6result in the same sorts of problems that leaving out a locking primitive
7would cause. This list is based on experiences reviewing such patches
8over a rather long period of time, but improvements are always welcome!
9 100. Is RCU being applied to a read-mostly situation? If the data
11 structure is updated more than about 10% of the time, then you
12 should strongly consider some other approach, unless detailed
13 performance measurements show that RCU is nonetheless the right
14 tool for the job. Yes, RCU does reduce read-side overhead by
15 increasing write-side overhead, which is exactly why normal uses
16 of RCU will do much more reading than updating.
17 18 Another exception is where performance is not an issue, and RCU
19 provides a simpler implementation. An example of this situation
20 is the dynamic NMI code in the Linux 2.6 kernel, at least on
21 architectures where NMIs are rare.
22 23 Yet another exception is where the low real-time latency of RCU's
24 read-side primitives is critically important.
25 261. Does the update code have proper mutual exclusion?
27 28 RCU does allow -readers- to run (almost) naked, but -writers- must
29 still use some sort of mutual exclusion, such as:
30 31 a. locking,
32 b. atomic operations, or
33 c. restricting updates to a single task.
34 35 If you choose #b, be prepared to describe how you have handled
36 memory barriers on weakly ordered machines (pretty much all of
37 them -- even x86 allows later loads to be reordered to precede
38 earlier stores), and be prepared to explain why this added
39 complexity is worthwhile. If you choose #c, be prepared to
40 explain how this single task does not become a major bottleneck on
41 big multiprocessor machines (for example, if the task is updating
42 information relating to itself that other tasks can read, there
43 by definition can be no bottleneck).
44 452. Do the RCU read-side critical sections make proper use of
46 rcu_read_lock() and friends? These primitives are needed
47 to prevent grace periods from ending prematurely, which
48 could result in data being unceremoniously freed out from
49 under your read-side code, which can greatly increase the
50 actuarial risk of your kernel.
51 52 As a rough rule of thumb, any dereference of an RCU-protected
53 pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
54 rcu_read_lock_sched(), or by the appropriate update-side lock.
55 Disabling of preemption can serve as rcu_read_lock_sched(), but
56 is less readable.
57 583. Does the update code tolerate concurrent accesses?
59 60 The whole point of RCU is to permit readers to run without
61 any locks or atomic operations. This means that readers will
62 be running while updates are in progress. There are a number
63 of ways to handle this concurrency, depending on the situation:
64 65 a. Use the RCU variants of the list and hlist update
66 primitives to add, remove, and replace elements on
67 an RCU-protected list. Alternatively, use the other
68 RCU-protected data structures that have been added to
69 the Linux kernel.
70 71 This is almost always the best approach.
72 73 b. Proceed as in (a) above, but also maintain per-element
74 locks (that are acquired by both readers and writers)
75 that guard per-element state. Of course, fields that
76 the readers refrain from accessing can be guarded by
77 some other lock acquired only by updaters, if desired.
78 79 This works quite well, also.
80 81 c. Make updates appear atomic to readers. For example,
82 pointer updates to properly aligned fields will
83 appear atomic, as will individual atomic primitives.
84 Sequences of perations performed under a lock will -not-
85 appear to be atomic to RCU readers, nor will sequences
86 of multiple atomic primitives.
87 88 This can work, but is starting to get a bit tricky.
89 90 d. Carefully order the updates and the reads so that
91 readers see valid data at all phases of the update.
92 This is often more difficult than it sounds, especially
93 given modern CPUs' tendency to reorder memory references.
94 One must usually liberally sprinkle memory barriers
95 (smp_wmb(), smp_rmb(), smp_mb()) through the code,
96 making it difficult to understand and to test.
97 98 It is usually better to group the changing data into
99 a separate structure, so that the change may be made
100 to appear atomic by updating a pointer to reference
101 a new structure containing updated values.
102 1034. Weakly ordered CPUs pose special challenges. Almost all CPUs
104 are weakly ordered -- even x86 CPUs allow later loads to be
105 reordered to precede earlier stores. RCU code must take all of
106 the following measures to prevent memory-corruption problems:
107 108 a. Readers must maintain proper ordering of their memory
109 accesses. The rcu_dereference() primitive ensures that
110 the CPU picks up the pointer before it picks up the data
111 that the pointer points to. This really is necessary
112 on Alpha CPUs. If you don't believe me, see:
113 114http://www.openvms.compaq.com/wizard/wiz_2637.html 115 116 The rcu_dereference() primitive is also an excellent
117 documentation aid, letting the person reading the code
118 know exactly which pointers are protected by RCU.
119 Please note that compilers can also reorder code, and
120 they are becoming increasingly aggressive about doing
121 just that. The rcu_dereference() primitive therefore
122 also prevents destructive compiler optimizations.
123 124 The rcu_dereference() primitive is used by the
125 various "_rcu()" list-traversal primitives, such
126 as the list_for_each_entry_rcu(). Note that it is
127 perfectly legal (if redundant) for update-side code to
128 use rcu_dereference() and the "_rcu()" list-traversal
129 primitives. This is particularly useful in code that
130 is common to readers and updaters. However, lockdep
131 will complain if you access rcu_dereference() outside
132 of an RCU read-side critical section. See lockdep.txt
133 to learn what to do about this.
134 135 Of course, neither rcu_dereference() nor the "_rcu()"
136 list-traversal primitives can substitute for a good
137 concurrency design coordinating among multiple updaters.
138 139 b. If the list macros are being used, the list_add_tail_rcu()
140 and list_add_rcu() primitives must be used in order
141 to prevent weakly ordered machines from misordering
142 structure initialization and pointer planting.
143 Similarly, if the hlist macros are being used, the
144 hlist_add_head_rcu() primitive is required.
145 146 c. If the list macros are being used, the list_del_rcu()
147 primitive must be used to keep list_del()'s pointer
148 poisoning from inflicting toxic effects on concurrent
149 readers. Similarly, if the hlist macros are being used,
150 the hlist_del_rcu() primitive is required.
151 152 The list_replace_rcu() and hlist_replace_rcu() primitives
153 may be used to replace an old structure with a new one
154 in their respective types of RCU-protected lists.
155 156 d. Rules similar to (4b) and (4c) apply to the "hlist_nulls"
157 type of RCU-protected linked lists.
158 159 e. Updates must ensure that initialization of a given
160 structure happens before pointers to that structure are
161 publicized. Use the rcu_assign_pointer() primitive
162 when publicizing a pointer to a structure that can
163 be traversed by an RCU read-side critical section.
164 1655. If call_rcu(), or a related primitive such as call_rcu_bh(),
166 call_rcu_sched(), or call_srcu() is used, the callback function
167 must be written to be called from softirq context. In particular,
168 it cannot block.
169 1706. Since synchronize_rcu() can block, it cannot be called from
171 any sort of irq context. The same rule applies for
172 synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
173 synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
174 synchronize_sched_expedite(), and synchronize_srcu_expedited().
175 176 The expedited forms of these primitives have the same semantics
177 as the non-expedited forms, but expediting is both expensive
178 and unfriendly to real-time workloads. Use of the expedited
179 primitives should be restricted to rare configuration-change
180 operations that would not normally be undertaken while a real-time
181 workload is running.
182 183 In particular, if you find yourself invoking one of the expedited
184 primitives repeatedly in a loop, please do everyone a favor:
185 Restructure your code so that it batches the updates, allowing
186 a single non-expedited primitive to cover the entire batch.
187 This will very likely be faster than the loop containing the
188 expedited primitive, and will be much much easier on the rest
189 of the system, especially to real-time workloads running on
190 the rest of the system.
191 192 In addition, it is illegal to call the expedited forms from
193 a CPU-hotplug notifier, or while holding a lock that is acquired
194 by a CPU-hotplug notifier. Failing to observe this restriction
195 will result in deadlock.
196 1977. If the updater uses call_rcu() or synchronize_rcu(), then the
198 corresponding readers must use rcu_read_lock() and
199 rcu_read_unlock(). If the updater uses call_rcu_bh() or
200 synchronize_rcu_bh(), then the corresponding readers must
201 use rcu_read_lock_bh() and rcu_read_unlock_bh(). If the
202 updater uses call_rcu_sched() or synchronize_sched(), then
203 the corresponding readers must disable preemption, possibly
204 by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
205 If the updater uses synchronize_srcu() or call_srcu(),
206 the the corresponding readers must use srcu_read_lock() and
207 srcu_read_unlock(), and with the same srcu_struct. The rules for
208 the expedited primitives are the same as for their non-expedited
209 counterparts. Mixing things up will result in confusion and
210 broken kernels.
211 212 One exception to this rule: rcu_read_lock() and rcu_read_unlock()
213 may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
214 in cases where local bottom halves are already known to be
215 disabled, for example, in irq or softirq context. Commenting
216 such cases is a must, of course! And the jury is still out on
217 whether the increased speed is worth it.
218 2198. Although synchronize_rcu() is slower than is call_rcu(), it
220 usually results in simpler code. So, unless update performance
221 is critically important or the updaters cannot block,
222 synchronize_rcu() should be used in preference to call_rcu().
223 224 An especially important property of the synchronize_rcu()
225 primitive is that it automatically self-limits: if grace periods
226 are delayed for whatever reason, then the synchronize_rcu()
227 primitive will correspondingly delay updates. In contrast,
228 code using call_rcu() should explicitly limit update rate in
229 cases where grace periods are delayed, as failing to do so can
230 result in excessive realtime latencies or even OOM conditions.
231 232 Ways of gaining this self-limiting property when using call_rcu()
233 include:
234 235 a. Keeping a count of the number of data-structure elements
236 used by the RCU-protected data structure, including
237 those waiting for a grace period to elapse. Enforce a
238 limit on this number, stalling updates as needed to allow
239 previously deferred frees to complete. Alternatively,
240 limit only the number awaiting deferred free rather than
241 the total number of elements.
242 243 One way to stall the updates is to acquire the update-side
244 mutex. (Don't try this with a spinlock -- other CPUs
245 spinning on the lock could prevent the grace period
246 from ever ending.) Another way to stall the updates
247 is for the updates to use a wrapper function around
248 the memory allocator, so that this wrapper function
249 simulates OOM when there is too much memory awaiting an
250 RCU grace period. There are of course many other
251 variations on this theme.
252 253 b. Limiting update rate. For example, if updates occur only
254 once per hour, then no explicit rate limiting is required,
255 unless your system is already badly broken. The dcache
256 subsystem takes this approach -- updates are guarded
257 by a global lock, limiting their rate.
258 259 c. Trusted update -- if updates can only be done manually by
260 superuser or some other trusted user, then it might not
261 be necessary to automatically limit them. The theory
262 here is that superuser already has lots of ways to crash
263 the machine.
264 265 d. Use call_rcu_bh() rather than call_rcu(), in order to take
266 advantage of call_rcu_bh()'s faster grace periods.
267 268 e. Periodically invoke synchronize_rcu(), permitting a limited
269 number of updates per grace period.
270 271 The same cautions apply to call_rcu_bh() and call_rcu_sched().
272 2739. All RCU list-traversal primitives, which include
274 rcu_dereference(), list_for_each_entry_rcu(),
275 list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
276 must be either within an RCU read-side critical section or
277 must be protected by appropriate update-side locks. RCU
278 read-side critical sections are delimited by rcu_read_lock()
279 and rcu_read_unlock(), or by similar primitives such as
280 rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
281 the matching rcu_dereference() primitive must be used in order
282 to keep lockdep happy, in this case, rcu_dereference_bh().
283 284 The reason that it is permissible to use RCU list-traversal
285 primitives when the update-side lock is held is that doing so
286 can be quite helpful in reducing code bloat when common code is
287 shared between readers and updaters. Additional primitives
288 are provided for this case, as discussed in lockdep.txt.
289 29010. Conversely, if you are in an RCU read-side critical section,
291 and you don't hold the appropriate update-side lock, you -must-
292 use the "_rcu()" variants of the list macros. Failing to do so
293 will break Alpha, cause aggressive compilers to generate bad code,
294 and confuse people trying to read your code.
295 29611. Note that synchronize_rcu() -only- guarantees to wait until
297 all currently executing rcu_read_lock()-protected RCU read-side
298 critical sections complete. It does -not- necessarily guarantee
299 that all currently running interrupts, NMIs, preempt_disable()
300 code, or idle loops will complete. Therefore, if you do not have
301 rcu_read_lock()-protected read-side critical sections, do -not-
302 use synchronize_rcu().
303 304 Similarly, disabling preemption is not an acceptable substitute
305 for rcu_read_lock(). Code that attempts to use preemption
306 disabling where it should be using rcu_read_lock() will break
307 in real-time kernel builds.
308 309 If you want to wait for interrupt handlers, NMI handlers, and
310 code under the influence of preempt_disable(), you instead
311 need to use synchronize_irq() or synchronize_sched().
312 313 This same limitation also applies to synchronize_rcu_bh()
314 and synchronize_srcu(), as well as to the asynchronous and
315 expedited forms of the three primitives, namely call_rcu(),
316 call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),
317 synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().
318 31912. Any lock acquired by an RCU callback must be acquired elsewhere
320 with softirq disabled, e.g., via spin_lock_irqsave(),
321 spin_lock_bh(), etc. Failing to disable irq on a given
322 acquisition of that lock will result in deadlock as soon as
323 the RCU softirq handler happens to run your RCU callback while
324 interrupting that acquisition's critical section.
325 32613. RCU callbacks can be and are executed in parallel. In many cases,
327 the callback code simply wrappers around kfree(), so that this
328 is not an issue (or, more accurately, to the extent that it is
329 an issue, the memory-allocator locking handles it). However,
330 if the callbacks do manipulate a shared data structure, they
331 must use whatever locking or other synchronization is required
332 to safely access and/or modify that data structure.
333 334 RCU callbacks are -usually- executed on the same CPU that executed
335 the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
336 but are by -no- means guaranteed to be. For example, if a given
337 CPU goes offline while having an RCU callback pending, then that
338 RCU callback will execute on some surviving CPU. (If this was
339 not the case, a self-spawning RCU callback would prevent the
340 victim CPU from ever going offline.)
341 34214. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
343 synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
344 may only be invoked from process context. Unlike other forms of
345 RCU, it -is- permissible to block in an SRCU read-side critical
346 section (demarked by srcu_read_lock() and srcu_read_unlock()),
347 hence the "SRCU": "sleepable RCU". Please note that if you
348 don't need to sleep in read-side critical sections, you should be
349 using RCU rather than SRCU, because RCU is almost always faster
350 and easier to use than is SRCU.
351 352 If you need to enter your read-side critical section in a
353 hardirq or exception handler, and then exit that same read-side
354 critical section in the task that was interrupted, then you need
355 to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid
356 the lockdep checking that would otherwise this practice illegal.
357 358 Also unlike other forms of RCU, explicit initialization
359 and cleanup is required via init_srcu_struct() and
360 cleanup_srcu_struct(). These are passed a "struct srcu_struct"
361 that defines the scope of a given SRCU domain. Once initialized,
362 the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
363 synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
364 A given synchronize_srcu() waits only for SRCU read-side critical
365 sections governed by srcu_read_lock() and srcu_read_unlock()
366 calls that have been passed the same srcu_struct. This property
367 is what makes sleeping read-side critical sections tolerable --
368 a given subsystem delays only its own updates, not those of other
369 subsystems using SRCU. Therefore, SRCU is less prone to OOM the
370 system than RCU would be if RCU's read-side critical sections
371 were permitted to sleep.
372 373 The ability to sleep in read-side critical sections does not
374 come for free. First, corresponding srcu_read_lock() and
375 srcu_read_unlock() calls must be passed the same srcu_struct.
376 Second, grace-period-detection overhead is amortized only
377 over those updates sharing a given srcu_struct, rather than
378 being globally amortized as they are for other forms of RCU.
379 Therefore, SRCU should be used in preference to rw_semaphore
380 only in extremely read-intensive situations, or in situations
381 requiring SRCU's read-side deadlock immunity or low read-side
382 realtime latency.
383 384 Note that, rcu_assign_pointer() relates to SRCU just as it does
385 to other forms of RCU.
386 38715. The whole point of call_rcu(), synchronize_rcu(), and friends
388 is to wait until all pre-existing readers have finished before
389 carrying out some otherwise-destructive operation. It is
390 therefore critically important to -first- remove any path
391 that readers can follow that could be affected by the
392 destructive operation, and -only- -then- invoke call_rcu(),
393 synchronize_rcu(), or friends.
394 395 Because these primitives only wait for pre-existing readers, it
396 is the caller's responsibility to guarantee that any subsequent
397 readers will execute safely.
398 39916. The various RCU read-side primitives do -not- necessarily contain
400 memory barriers. You should therefore plan for the CPU
401 and the compiler to freely reorder code into and out of RCU
402 read-side critical sections. It is the responsibility of the
403 RCU update-side primitives to deal with this.
404 40517. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and
406 the __rcu sparse checks to validate your RCU code. These
407 can help find problems as follows:
408 409 CONFIG_PROVE_RCU: check that accesses to RCU-protected data
410 structures are carried out under the proper RCU
411 read-side critical section, while holding the right
412 combination of locks, or whatever other conditions
413 are appropriate.
414 415 CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the
416 same object to call_rcu() (or friends) before an RCU
417 grace period has elapsed since the last time that you
418 passed that same object to call_rcu() (or friends).
419 420 __rcu sparse checks: tag the pointer to the RCU-protected data
421 structure with __rcu, and sparse will warn you if you
422 access that pointer without the services of one of the
423 variants of rcu_dereference().
424 425 These debugging aids can help you find problems that are
426 otherwise extremely difficult to spot.
427